1. Field of The Invention
The present invention relates generally to a suspension control system for an automotive vehicle which utilizes variable damping force shock absorbers for suppressing bouncing, pitching, and rolling motions of a vehicle body. More particularly, the invention is directed to a suspension control system which is operable to modify damping coefficients of shock absorbers for attenuating bouncing motion caused by high frequency vibrations transmitted from a road surface effectively while securing riding comfort.
2. Description of The Prior Art
Japanese Utility Model First Publication No. 63-93203 discloses an automotive suspension control system which utilizes variable damping force shock absorbers. This suspension control system is operable to detect vertical speed of a vehicle body and relative speed between the vehicle body and a road wheel as control parameters for modifying damping force characteristics of the shock absorbers. When the vertical speed is in an upward direction and the relative speed is increased or when the vertical speed is in a downward direction and the relative speed is decreased, the shock absorbers of all wheels are controlled to assume softer damping force characteristics respectively.
However, in such a prior art suspension control system, when vehicle attitude change of bouncing, pitching, and rolling motions occurs, causing an inertial moment with respect to the center of gravity of a vehicle body to act on a sprung mass, damping forces of shock absorbers are modified to assume harder damping force characteristics for suppressing the pitching and rolling motions greatly to secure traveling stability. With this harder damping force characteristic modification, the damping force against the bouncing motion tends to be undesirably great. In other words, the damping force becomes greater than necessary for suppressing the bouncing motion appropriately while assuring passenger comfort.
Additionally. Japanese Patent First Publication No. 61-163011 discloses a suspension control system similar to the above discussed conventional system. This suspension control system is responsive to high frequency vibrations input from a road surface to control damping force to exhibit harder or softer damping force characteristics dependent upon a direction of the vibrations. However, such a conventional suspension control system encounters a drawback in that a response rate of damping force modification tends to be delayed due to time, and for processing a control program and operation of an actuator varying damping force of a shock absorber. Therefore, while vibration attenuation is improved against low frequency input to a vehicle body, riding comfort tends to be degraded when a vehicle traveling on an uneven road causing high frequency vibrations to be transmitted to the vehicle body.